A method for producing acetic acid by continuously reacting methanol with carbon monoxide using a rhodium catalyst and methyl iodide in the presence of water is one of the most excellent industrial production methods of acetic acid (refer to Patent Document 1). The original method, however, requires a great deal of energy to remove water in a purification process, since the resulting reaction mixture has a high water content. If the water content of the reaction mixture is reduced, the rhodium catalyst is inactivated and the productivity of acetic acid decreases.
As a possible solution to reduce the water content of the reaction mixture without inactivating the rhodium catalyst, propose improved methods in which a catalyst stabilizer such as an iodide salt is added to the reaction system were proposed (refer to Patent Documents 2 and 3). The improvements in these methods serve to stabilize the rhodium catalyst and to reduce byproducts such as carbon dioxide and propionic acid. The methods, however, invite increase in by-produced carbonyl-containing reducing impurities to thereby decrease the quality of produced acetic acid. These impurities include acetaldehyde, as well as crotonaldehyde, 2-ethylcrotonaldehyde, and other products of consecutive reactions, such as aldol condensation, of acetaldehyde. In addition, the methods also invite increase in by-produced alkyl iodides, such as hexyl iodide derived from acetaldehyde. Such alkyl iodides will degrade palladium catalysts used in the production of vinyl acetate from acetic acid and ethylene.
Granted Japanese Patent No. 3244385 discloses a method for producing high-purity acetic acid by continuously reacting methanol with carbon monoxide in the presence of a rhodium catalyst, an iodide salt, and methyl iodide, in which the reaction is carried out while maintaining the acetaldehyde content of the reaction mixture to 400 ppm or less. This document mentions a specific method of maintaining the acetaldehyde content of the reaction mixture to 400 ppm or less by removing acetaldehyde from a process mixture circulating to a reactor, but it lacks a detailed description on inhibition of aldehyde production.
PCT Japanese Translation Patent Publication No. 2003-508363 discloses a technique for reducing impurities by controlling the hydrogen partial pressure within the range of 0.1 to 4 psia (0.7 to 27.6 kPa) at a low water content in a method for producing acetic acid by reacting methanol with carbon monoxide in the presence of a rhodium catalyst. Such an extremely low hydrogen partial pressure generally fails to ensure a sufficiently high catalytic activity in the production of acetic acid. Japanese Examined Patent Application Publication (JP-B) No. 8-5839 mentions that the production rate of acetic acid decreases at a low hydrogen partial pressure of 40 psi (276 kPa) or less. In addition, highly pure carbon monoxide containing a minimum amount of hydrogen is required for controlling the hydrogen partial pressure at such a very low level. The production of highly pure carbon monoxide requires enhanced facilities for purification and invites increased cost. The present inventors have verified that the acetaldehyde content and amounts of other byproducts cannot be sufficiently reduced even by this method.
PCT International Publication Number WO 2004/60846 discloses a process for producing acetic acid in which acetic acid is produced at a production rate (STY) of 15 mol/L·hr or more at a water content of 2 percent by weight or less and a rhodium content of 1000 ppm or more. According to this document, a reaction for producing methane (CH3OH+H2+→CH4+H2O) takes priority over a water gas shift reaction (CO+H2O→CO2+H2), and water forms and accumulates in the reaction system at a water content in the reaction system of 5 percent by weight or less. To avoid this, the method uses methyl acetate in the reaction system for chemically controlling the water content. The added methyl acetate comes in contact with water in the system and is hydrolyzed into acetic acid and methanol. The resulting methanol is used as a raw material for the production of acetic acid. The increased rhodium content in the system, however, increases not only the production rate of acetic acid but also that of by-produced acetaldehyde. Acetic acid and acetaldehyde increase proportionally with increase in rhodium content. Specifically, acetaldehyde increases substantially proportionally with increase in acetic acid. At a low water content, the rate of aqueous gas shift reaction decreases to thereby decrease the hydrogen partial pressure. At such a low hydrogen partial pressure, the rates of hydrogenation reactions of acetaldehyde (e.g., CH3CHO+H2+CH3CH2OH) decrease to thereby increase the acetaldehyde content of the reaction mixture and increase the rate of condensation reactions of acetaldehyde. Consequently, reducing substances such as crotonaldehyde and 2-ethylcrotonaldehyde as products consecutive reactions of acetaldehyde increase to thereby cause poor results in a potassium permanganate test of the product acetic acid. The present inventors have verified the above facts as a result of investigations. In contrast, at a somewhat high hydrogen partial pressure, e.g., at a hydrogen partial pressure of 11 to 14 psi (75.8 to 96.5 kPa) as described in PCT International Publication Number WO 2004/60846, water formed upon by-production of methane must be removed. This requires extra energy and an extra agent for removing water and thereby reduces the production efficiency. This also increases the by-production of formic acid that is a reducing substance and causes poor results in a potassium permanganate test.
Applied Homogeneous Catalysis with Organometallic Compounds (2nd Edition) (2002), Volume 1, 104-136 (Celanese) relates to techniques for synthesizing acetic acid at a low water content and mentions that the stability of a rhodium complex is increased by adding lithium iodide, that the carbonylation reaction rate markedly increases with an increasing methyl acetate content at low water contents, and that the carbonylation reaction rate is increased by the action of the iodide salt. This document, however, lacks the description about the control of acetaldehyde content. The present inventors have found that while the addition of lithium iodide for improving the stability of a rhodium complex increases the production rate of acetic acid, it further increases the production rate of acetaldehyde.
JP-A No. 6-40999 discloses a method for producing acetic acid, in which a reaction is carried out while keeping the water content to about 10 percent by weight or less and the methyl acetate content to 2 percent by weight or more, and the resulting reaction mixture is distilled. This document mentions that the by-production of propionic acid decreases with an increasing methyl acetate content, and the propionic acid content of the reaction mixture becomes less than 500 ppm at a methyl acetate content of 2 percent by weight. As is described above, however, the reaction rate of acetaldehyde hydrogenation decreases under a low hydrogen partial pressure at a low water content, the acetaldehyde content of the reaction mixture increases to thereby increase the reaction rate of acetaldehyde condensation, although the production level of propionic acid due to carbonylation of ethanol decreases. As a result, reaction products of acetaldehyde condensation increase to thereby impair the quality of acetic acid. To achieve a process for efficiently producing acetic acid which saves resources and energy, the by-production of both acetaldehyde and consecutive reaction products of acetaldehyde, such as condensation reaction products, propionic acid, and hexyl iodide, must be effectively inhibited.
PCT International Publication Number WO 2002/62740 discloses a low-energy process for producing acetic acid using two or less distillation columns, in which a product flow contains propionic acid impurities at low level, and the level of aldehyde impurities in the product flow is controlled through a technique selected from (i) maintaining the hydrogen partial pressure in the reactor to less than about 6 psia (41.3 kPa) at a total pressure of from about 15 to 40 atmospheres (1.5 to 4 MPa) in the reactor; or (ii) maintaining the methyl iodide content of the reaction mixture of less than about 5 weight percent; or (iii) removing aldehyde impurities. The reduction in hydrogen partial pressure, however, increases by-production of consecutive reaction products of acetaldehyde including acetaldehyde condensation reaction products to thereby impair the quality of acetic acid, as mentioned above. The reduction in methyl iodide content reduces the production rates of not only acetaldehyde but also acetic acid, as described in Example of this document (Table 3). Namely, it reduces the production efficiency of acetic acid and is industrially and economically undesirable.
Patent Document 1: Japanese Examined Patent Application Publication (JP-B) No. 47-3334)
Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 60-54334
Patent Document 3: JP-A No. 60-239434
Patent Document 4: Granted Japanese Patent No. 3244385
Patent Document 5: PCT Japanese Translation Patent Publication No. 2003-508363
Patent Document 6: Japanese Examined Patent Application Publication (JP-B) No. 8-5839
Patent Document 7: PCT International Publication Number WO 2004/60846
Patent Document 8: JP-A No. 6-40999
Patent Document 9: PCT International Publication Number WO 2002/62740
Non-Patent Document 1: Applied Homogeneous Catalysis with Organometallic Compounds (2nd Edition) (2002), Volume 1, 104-136 (Celanese)